Continuous phosphorus removal in electric-arc steelmaking



CONTINUOUS PHOSPHORUS REMOVAL IN ELECTRIC-ARC STEELMAKING Filed June 2, 1967 g- 1970 R. LITTLEWOOD ETAL 3 Sheets-Sheet l PRESENT INVENTION FIG. I

MAKE UP OF 2O FEEPZMOZOQ wDmOImwOIa P U S R M WIN GfiOL A HA GEM L PV A 8 O L W 6 OM R N HE I PR Q O H P S O H P E D fiz mo ZO I. .ZwOZOO wDmOTEmOIQ T|ME- TAP FIG. 2 (PRIOR ART) 5 E M l n G F H 0 0 a .All C wmmmowov mmpzmmaimk Ih m TIME F IG 3 (PRIOR ART) 0 O 8 2 2 Al E $589 MEDZEQEE 12m TlME- FIG.7

(PRIOR ART) INVENTORS.

ROY LITTLEWOOD &

BY GORDON A. ROEDER 4 g Mulollaml ATTORNEYS 4, 1970 R. LITTLEWOOD ETAL CONTINUOUS PHOSPHORUS REMOVAL IN ELECTRIC-ARC STEELMAKING 5 Sheets-Sheet 2 Filed June 2, 1967 0 w E m E S K w U T R 60 L A E OT N m HE M 8 S o w N ITS PI 0 WE M E a m WA T T Sm N m E0 PW 6 M M h T mm O T I W OWE O O O O 4 LIME ADDITIONS 3 2 I III 2 x m: duh. 55,233 08 53 30 TIME AFTER POWER-ON (MINUTES)" FIG. 9

INVENTORS- ROY LITTLEWOOD 8| GORDON A. ROEDER BY 444;, 544 ii 7x14011014 ATTO R NEYS 1970 R. LITTLEWOOD ETAL 3,523,020

CONTINUOUS PHOSPHORUS REMOVAL IN ELECTRIC-ARC STEELMAKING Filed June 2, 1967 5 Sheets-Sheet 5 FIG. IO

9 T 3 E 00 V-RATIO 3 T 2E T o 200 I z D 0C 0: 7 3 I00 CL INITIAL COMPLETE 8 MELT OUT MELT OUT L o o 0 30 6O 90 I20 I50 TIME (MINUTES) FIG. ll

O TPER|0D OF{ o CONTINUOUS TIME-T CHARGING V-RATIO R, PHOSPHORUS PARTITION RATIO INVENTORS. ROY LITTLEWOOD 8 GORDON A. ROEDER BY 4 5 4M ATTORNEYS United States Patent Ofice 3,523,020 Patented Aug. 4, 1970 I111. c1. cm 5752; C21b 3/02 US. Cl. 75-11 13 Claims ABSTRACT OF THE DISCLOSURE This disclosure relates to a method of dephosphorizing iron in an electric-arc steelmaking process. As phosphorus-containing iron-bearing material is being fed continuously into a furnace, slagging ingredients, principally lime, are fed substantially continuously into the furnace while melting the same. The feeding rate of lime is increased during the continuous charging of iron-bearing material such that the ratio of calcium oxide to silicon dioxide of the slag rises. In this manner, phosphorus is removed from the metal bath at a substantially constant and higher rate than it is being added through the continuous charging of iron-bearing material.

SUMMARY OF THE INVENTION This invention relates to continuous removal of phos phorus from steel in electric-arc steelmaking while a technique of continuous charging of iron-bearing material which contains phosphorus is being employed.

This invention is particularly adapted for use with the continuous feeding of reduced iron pellets and/or scrap into an electric-arc furnace in an operation in which the refining is done substantially continuously and at the same time as the melting process.

The invention more particularly is an effective method of removing phosphorus continuously from a steel bath during continuous charging of iron-bearing material containing phosphorus such that the phosphorus removal rate is held substantially constant, so that, nothwithstanding the feeding of phosphorus-bearing charge material to the bath, the phosphorus content of the metal is caused to fall continually, reaching the required low level at the end of continuous charging. The method involves controlling slag composition and bath temperature during continuous charging. Optimum phosphorus removal conditions are created by adjusting the FeO concentration in the slag to about 15% by weight (i.e., from 6% to 24%) and by maintaining the temperature of the molten bath in the range 2550 F. to 2850 F., while an active boil is maintained in the bath by ensuring that the combined residual oxygen in the iron-bearing feed material is between 0.1% and 1.75% oxygen (preferably 0.7% to 1.2%), or by means of oxygen lancing.

Furthermore, the invention is directed to adjusting slag V-ratio (percent CaO/ percent SiO by weight in the slag) so that V-ratio increases during melting and is at a maximum at the completion of melting, which is coincident with the completion of continuous charging. This procedure ensures that phosphorus in the metal may be removed continuously during continuous charging and at a rate higher than the rate at which phosphorus is added in the continuously charged iron-bearing material. This adjustment of V-ratio permits the rate at which phosphorus is removed from the metal to be maintained almost constant even though the concentration of phosphorus in the metal is falling during continuous charging. The adjustment of V-ratio is attained by adding CaO to the slag continuously at an increasing rate such that the rate of addition is substantially greater in the latter portion of continuous charging than in the former.

In a previous application entitled Improvements in Electric-Arc steelmaking by Siba'kin et al., US. Ser. No. 571,837, filed Aug. 11, 1966, a method of steelmaking in electric-arc furnaces was described in which the majority of the iron-bearing charge is continuously fed to the arc region of the furnace after the partial melting of a small initial charge. This method of charging resulted in substantial improvements to furnace productivity (tons of steel produced per unit time) for the following reasons: Time and heat loss associated with scrap recharging was eliminated; the conventional refining period was found to be unnecessary; and a higher power level was able to be achieved since there was a steady demand for energy during continuous charging. However, it is difficult to apply this technique of continuous charging when the ironbearing material contains phosphorus, using prior art methods of phosphorus removal, without losing part of the productivity advantage of the continuous charging technique.

In one known prior art method of phosphorus removal, the whole of the iron-bearing charge is first melted and then a dephosphorizing slag is made up in the furnace. The necessary characteristics of this slag are known: it should be fluid, oxidizing and basic; it should contain a high concentration of lime; and the optimum iron oxide content is about 15% by weight. Other necessary conditions for dephosphorization are known to be a relatively low temperature and efficient mixing of slag and metal. After sufficient time has elapsed to reach the desired low phosphorus content of steel, the phosphorus-containing slag is removed. Further adjustments in the steel composition are then made (if so desired), and the temperature is then caused to rise rapidly. At this point, tapping takes place.

In applying this prior art to steelmaking using continuous charging of iron-bearing material containing phosphorus, it is necessary to carry out the dephosphorization procedure described in the previous paragraph after the continuous charging is completed. This wastes electrical energy and time, and productivity decreases.

Previously, many methods have been proposed for the removal of phosphorus from molten steel. These have generally been associated with steelmaking processes involving batch charging and have been essentially variations of the basic dephosphorization process described above. In some cases, continuous feeding of dephosphorizing ingredient has been suggested, but this has been solely to ensure that highly dephosphorizing conditions persist at the slag-metal interface throughout a phosphorus removal stage of refining. In many cases, for example, injection of powdered dephosphorizing agents in a gaseous carried has been suggested as a convenient way of feeding materials to the bath and ensuring that they reach the slag-metal interface where their effectiveness is greatest. Such suggestions have often been associated with pneumatic steelmaking processes where a jet of air or oxygen is used as the primary steelmaking agent. For example, U.S. Pat. No. 2,671,018 to R. F. Graef entitled Process for the Production of Basic Bessemer Steel Low in Nitrogen describes the injection of powdered lime with the blast of a basic Bessemer converter. The lime acts as a dephosphorizing agent, and the object of the invention is to use this injection method as a means of producing a low nitrogen steel. In the LDAC process, a modification of the LD process or top blown converter, powdered lime is injected through the oxygen lance to promote basic conditions in the slag required, inter alia, for phosphorus removal. Chater and Cook (Steel Times, May 29, 1964, pp. 718-21) described injection methods whereby dephosphorizing reactions can be carried out by simultaneously injecting several agents down a specially designed lance. All these proposals are concerned with methods of introducing slag-making materials in an operation in which steel is made by a pneumatic batch process. In all such operations, it is possible to relate the quantities of dephosphorizing materials required to the composition of the metal to be refined. For example, U.S. Pat. No. 2,804,385 to R. F. Gracf entitled Method of Refining Phosphorus-Containing Pig Iron describes specifically a two-slag process in which lime additions to the second slag are determined with reference to the phosphorus content of the pig iron to be refined. The features by which the present invention is distinguished from the foregoing suggestions are that the above proposals refer to pneumatic batch processes, and that continuous charging of iron-bearing material concurrently with slagforming materials according to the critical process parameters of the present invention is not involved.

A prior description of a pneumatic process for refining pig iron in which substantially continuous addition of material are made is noted in Oelsen et al. Pat. No. 2,815,274. In a basic Bessemer (or Thomas) converter, a small charge of pig iron and lime is initially refined by blowing, and then subsequent charging operations are carried out by substantially continuous additions of pig iron and lime. There are several features which distinguish this proposal from the present invention. The Oelsen et al. process is directed toward blowing processes, specifically the basic Bessemer, which rely on the heat involved in chemical reactions alone for attainment of tapping temperature, and in which both temperature and melt composition normally vary rapidly with time. An important feature of the Oelsen et al. process is to refine completely the small initial charge and then to maintain temperature and bath composition substantially constant while making subsequent additions of pig iron and lime. This distinguishes the said process from our method, where refining progresses and bath temperature rises throughout the period of continuous charging. While the Oelsen et al. patent mentions that amounts of lime added are dependent on the metalloid content of the pig iron,

' no specific directives are given on how to manipulate lime additions to control phosphorus content in the final product.

As illustrations of the difiiculties associated with combining prior art methods of phosphorus removal with continuous charging in an electric furnace, the following experimental heats, carried out by ourselves and others skilled in the art, are cited.

In experiments carried out to investigate the possibility of melting phosphorus-bearing sponge iron in an electricarc furnace to produce steel of low-phosphorus content,

it was found that dilution of the resulting phosphorusbearing melt with low-phosphorus sponge iron or scrap steel was an ineffective method of reaching a low-phosphorus content (less than 0.02%), even when melting and dilution were carried out under the optimum dephosphorizing conditions described above. For example, in experiments in a 25-ton electric-arc furnace, 26,800 lbs. of sponge iron containing 0.08% phosphorus were diluted with 25,600 lbs. of low-phosphorus steel scrap and gave a cast steel product containing 0.037% phosphorus. 14,000 lbs. of 0.08% phosphorus sponge iron diluted with 10,500 lbs. low-phosphorus scrap and 28,000 lbs. phosphorus-free sponge iron gave a product containing 0.042% phosphorus. Continuously charging of the charge (balance low-phosphorus scrap) using 0.08% phosphorus sponge iron also led to a high-phosphorus content in the final product (e.g., 0.067% phosphorus in one experimental heat in which 14,700 lbs. scrap and 38,100 lbs. of sponge iron were melted). In further experiments a dephosphorizing slag was made up and removed at intervals during continuous charging, which was interrupted during these operations. Phosphorus levels of 0.02% in the steel product were obtained by this method, but the wastage of time and use of considerable electrical energy caused by the slag treatment op erations led to unacceptably poor productivity and power consumption. Better productivity was achieved when dephosphorizing conditions were obtained by periodic additions of lime made during continuous charging, with slag being run off at intervals, but phosphorus removal was unsatisfactory. For example, in three experiments in a 25-ton electric-arc furnace with a proportion of 0.08% phosphorus sponge iron in the charge (the balance being low-phosphorus scrap), the average phosphorus level in the final steel product was 0.04%. In five heats in a 75-ton arc furnace carried out under similar conditions, using sponge iron containing 0.054% phosphorus, the average phosphorus content of the product was 0.028%, indicating once again that very little phosphorus was removed from the steel by this technique.

The combination of prior art methods of phosphorus removal with continuous charging of phosphorus-bearing iron material and melting in an electric-arc furnace is not entirely satisfactory. We have found that a separate refining stage can be obviated and, surprisingly, the phosphorus content of the metal 'bath reduced, notwithstanding the cumulative increase of total phosphorus added to the furnace in continuously charged material, by the process of the present invention. The new method allows phosphorus refining to be carried out concurrently with continuous charging and with the other refining operations of steelmaking which are already carried out during continuous charging in the manner described in the previous application of Sibakin et al. A heat of steel made from phosphorus-bearing charge material, according to the teachings of the present invention, would then, except for standard alloying additions, be ready for tapping im mediately at the end of continuous charging.

Accordingly, it is an object of this invention to remove phosphorus continuously in an electric-arc steelmaking process with high efficiency while substantially continuously feeding the furnace with phosphorus-containing iron-bearing material. An important object of the invention is to ensure that phosphorus is removed at a substantially constant rate over the Whole period of con tinuous charging. We have found that this leads to good productivity combined with a product of low-phosphorus content.

A further object of the invention is to achieve a high efiiciency of phosphorus removal by controlling the temperature of the bath to keep it below 2850 F. throughout continuous charging and to achieve such temperature control by means of fine adjustments to the charging rate of sponge iron, keeping the power input to the furnace constant, preferably at the maximum. The bath temperature may be allowed to rise gradually to 2850 F. during continuous charging, or else to rise to that value at an early stage and be held there by manipulation of sponge iron feed rate, while keeping the power supply to the furnace at the maximum.

FIG. 5 is a graph of the phosphorus concentration of the metal versus time for the process of the invention, in which phosphorus is continuously removed during continuous charging.

FIG. 6 is a graph of the bath temperature versus time Another object of the invention is to keep the Foo 5 for the process of the invention. content of the slag near the optimum 15% for phos- FIG. 7 is a graph of the total amount of lime added phorization, as well as to control the amount of carbon to the furnace versus time for the process of the invention. boil i the bath, y keeping the amount of Combined FIG. 8 isa comparative graph showing the total amount residual oxygen present in the iron-bearing feed material 10 of phosphorus added to the furnace in the charge, and at between 0.7% and 1.2% by weight of the feed matealso the amount of phosphorus in the metal, 'both versus rial, or by lancing the bath with oxygen. time, for the process of the invention and summarizes Another Object of the invention iS t promote efiicient data taken in a typical heat of steel made according to dephosphorization by ensuring rapid dissolution of lime the teachings of the invention. and Other Slag-forming materials and generating a flow FIG. 9 is an example of the general case shown in of dephosphorizing slag across the "bath. FIG. 7, and summarizes data on lime additions for the Another object of the invention is to adjust the V-ratio a h t as FIG, 8, of the slag so that the rate of phosphorus remov l f FIG. 10 shows the variations of V-ratio in the slag and the iron remains substantially constant throughout the hosphorus partition ratio in the above heat of steel. process, even though the concentration of the phosphorus FIG, 11 i a graph showing the preferred ranges of in the metal is decreasing. V-ratio and R-ratio versus time for the process of the Another Object of the invention is to ensure that the invention, based on data from experimental heats using furnace operates with maximum efficiency by causing th variations of the teachings of the present invention. arcs to submerge in the slag. Under normal conditions It can be seen that FIGS. 5 to 10 relate specifically with a highly basic slag, such as that needed for deto the process of this invention. FIGS. 8 to 10 summarize phosphorization in an electric-arc furnace, the arcs strike data taken in a typical heat of steel made according to the tw n the electrodes and the Slag. and the furnace Walls teachings of the invention. In this heat, the initial charge and ro f r easily damaged y radiation m h arc was 9,000 lbs. of scrap iron and 12,760 lbs. of phosphoruswhen the furnace is running at fu l poW r. C ntinu bearing sponge iron pellets. Sufiicient carbon was added charging, With feed directed to the are flare TegiOHS, to ensureavigorous boil throughout continuous charging cools the slag in this area, lowering its conductance and of more phosphorus-bearing ponge hi h h d a ho causes the arcs to submerge. Thus, melting can proceed phorus concentration of .08% by weight. This continuous at maXimtlm power level even With a highly basic Slag charging was begun 37 minutes after the electrode power without damaging the furnace. was turned on, and the charging continued for 85 minutes. In summary, this invention most generally s a m th A sample of the melt was taken at 55 minutes, 88 minutes for dephosphorizing iron in an electric-arc steelmaking and 104 minutes after the power wa turned on, During process in which the phosphorus-containing iron-bearing the continuous charging period, the phosphorus content material is being fed continuously to the furnace. The of the metal in the bath fell from 0.024% to 0.007%, notmethod involves feeding slagging materials (principa ly withstanding the addition of 31,000 lbs, of sponge iron lime) preferably into the arc flare region of the furnace 40 containing 08% phosphorus during the same period. Iron bath along with the iron-bearing material in a substanoxide (FeO) content of the slag was held between 9.4% tially continuous manner, and such that the FeO content and 15.3% and the bath temperature kept below 2850 of the slag is kept near 15%, the V-ratio is made to rise F. throughout the combined continuous charging and rethroughout the process, the temperature is kept between fining operation. Lime powder was added continuously. 2550 F. and 2850" F., and so that as a result the phos- Slag was flushed at intervals from the furnace. FIG. 8 phorus is removed r the metal at a substantially indicates how the phosphorus content of the metal bath Constant rate throughout the p fell at a substantially constant rate during the continuous The Preferred embodiment of the inventiofl W111 be charging and refining operation. FIG. 9 shows the cumula- Presehtly descfibed With reference to the drawlngstive amount of lime added to the furnace and shows that In the dfawmgsi the rate was doubled when continuous charging was 1 a graph of charg? the furnace,versus half completed. FIG. 10 indicates how the acceleration for contmupus feed. elecmciarc process in lime feed rate enabled the phosphorus content of the Showmg the d1fie.rent for a process Wlth a .sgtjarate bath to be continually reduced during continuous chargphosghorus. refinmg penod the process. of thls .mven' ing. The accelerated lime additions caused the V-ratio of non, 1n wh1ch phosphorus 1s removed during continuous the 1 t hich in r S d th h h charging, and there is no separate refining period. ag 0 W c ea 6 e p Orus par 1 Ion FIG. 2 is a graph of the phosphorus concentration of rat) R (percent P205 slag/perfient P metal) from the metal versus time for the process with the separate to 220 over the contlnuous chargmg- Phos refining perimi phorus was thus progressively removed from the metal FIG. 3 is a graph of the bath temperature versus time 60 hathfor the process with the Separate fi i i The following table gives further details of the opera- FIG. 4 is a graph of the total amount of lime added tion of this heat:

Phosphorus Rate of Total partition phos. weight Weight Pllos. Total ratio, R, removal Total of phos. of phos. content weight percent; Percent over each pellets (led in remaining of pllos. P2O5inslag/ 0210/ time Time after powerin bath, pellets,) in metal metal, removed, percent percent interval, 011. mins. lbs. lbs. bath. lbs. percent lbs. P in metal S102 lbs/thins.

to the furnace versus time for the process with the separate refining period.

FIG. 1 illustrates the most important advantage of the process of this invention over that of the prior art using a separate phosphorus refining period. The time between the dotted lines is saved in this invention since the separate refining period is eliminated. This is important since it increases the productivity of the furnace (tons of steel produced per unit time).

FIGS. 2, 3 and 4 illustrate the prior art practice in which there is a refining period and contrast with FIGS. 5, 6 and 7, respectively, which illustrate the invention. FIG. 2 shows the amount of phosphorus in the metal steadily increasing to a maximum during continuous charging of the furnace, since no phosphorus is being removed at that time. Then after the dephosphorizing slag is made up, the phosphorus content of the metal falls as refining takes place. In contrast, FIG. 5 shows the amount of phosphorus in metal gradually decreasing throughout the continuous feeding cycle, even though phosphorus-containing material is being fed into the furnace continuously. FIGS. 3 and 6 compare the bath temperature histories for the two processes. In the separate refining period case, it would be necessary to maintain bath temperature below 2850 F. until after the end of the refining period. In order to accomplish this, the power to the electrodes would have to be cut back during refining. After the refining period, the power would again be raised to bring the temperature rapidly up to the tapping temperature. In the continuous phosphorus removal case of this invention, however, the temperature gradually rises during the charging period, and finally rises rapidly to the tapping temperature. The furnace is operated throughout at maximum power, bath temperature being kept below 2850 F. by varying the rate of addition of charge material. The furnace, therefore, operates throughout under conditions giving maximum melting rate of charge. At the end of the charging period, which coincides with the end of phosphorus removal, the temperature is allowed to increase automatically, at maximum power and, therefore, in the shortest possible time, until the tapping temperature is reached. Conditions throughout are conducive to minimum heat time and, therefore, maximum productivity.

FIGS. 4 and 7 compare the lime additions in the two cases. In the prior art, lime is added with the initial charge, and then again at the beginning of the refining period, to make the dephosphorizing slag. There would be a time delay before the lime dissolved in the slag and dephosphorizing conditions were established (FIG. 2) and carbon and/or iron oxide additions might be needed to promote turbulence in the bath by initiating a carbon boil and achieving the conditions required for dephosphorization.

In this invention, however, as shown in FIGS. 7 and 9, lime is added substantially continuously at an increasing rate during the charging period. The term during, as used in this application, is intended to mean throughout, i.e. continuously, or from time to time, i.e. at intervals. The controlled increase in lime addition rate is crucial to the success of the new process. The dephosphorizing power of the slag is caused to rise during continuous charging so that the required decrease in phosphorus content of the molten metal is achieved, notwithstanding the continual addition of phosphorus-containing material. Although it may be expedient to retain the phos phorus-containing slag in the furnace during the continuous charging period, it may be preferred to flush the slag at intervals, as in the foregoing example, or continuously, from points remote from regions of introduction of slagforming materials. Regions particularly suitable for introduction of slag-forming materials have been found to be the arc flare regions where high turbulence occurs. The feeding rate of lime is chosen so as to ensure that the V-ratios and R-ratios are kept within the allowable ranges shown in FIG. 11, i.e. the R-ratio varies from 28 to as high as 350, and the V-ratio varies from its initial value at 2.0 to 2.4 and ends at 2.9 to 32.

FIG. 8 illustrates clearly a major advantage in the process of this invention. Even though the phosphorus is continually added to the furnace, as shown by the upper line, the lower line strikingly shows that the amount of phosphorus in the metal bath steadily decreases.

In summary, the present invention provides a number of advantages. It allows phosphorus refining to be carried out concurrently with continuous charging. Thus, it extends the productivity benefits of a continuous charging process in electric furnace steelmaking to encompass ironbearing charge materials containing phosphorus as well as those which are phosphorus free. Previously known techniques of dephosphorization, when used in conjunction with continuous charging, are either ineffective or give rise to unacceptable losses in productivity. Operating the process with all factors set at optimum levels and controlling phosphorus removal rate by adjusting the V-ratio has the additional advantage of simplicity, since it is relatively easy to install continuous lime feeding and feed rate controlling equipment in conjunction with installations for continuous charging of iron-bearing materials.

Having thus explained the broad teachings of our invention and its advantages, we wish it to be known that We do not care to be liimted by all of the foregoing but only by the claims appended.

We claim:

1. A method of removing phosphorus continuously from a metal bath in electric-arc steelmaking during continuous charging to a furnace of phosphorus-containing iron-bearing material comprising feeding slagging ingredients, principally lime, substantially continuously to the furnace and melting same, and increasing the feeding rate of lime during the continuous charging of iron-bearing material such that the V-ratio (percent CaO/percent SiO by weight in the slag) of the slag rises during melting and is at a maximum at the completion of melting, whereby phosphorus is removed from the metal bath at a substantially constant and higher rate than it is being added through the continuous charging of iron-bearing material.

2. The method of claim 1 in which an initial charge of phosphorus-containing iron-bearing material together with slagging ingredients is put into the furnace before power is applied to the electrodes.

3. The method of claim 1 in which the V-ratio of the slag is caused to rise from between 2.0 and 2.4 at the beginning of continuous charging to between 2.9 and 3.2 before tapping.

4. The method of claim 1 in which the partition ratio is caused to rise from about 28 to about 350 during con tinuous charging.

5. The method of claim 1 in which the feed rate of slagging ingredients and of iron-bearing material is controlled so that the FeO concentration in the slag is between 6% and 24% and the bath temperature is maintained between 2550" F. and 2850 F. during continuous charging.

6. The method of claim 5 in which control of FeO concentration and temperature is partially or substantially achieved by lancing the bath with oxygen.

7. The method of claim 1 in which the feed rate of the iron-bearing material is adjusted so that the temperature of the bath is maintained between 2550 F. and 2850 F. while the power to the electrodes remains substantially constant at or near maximum.

8. The method of claim 1 in which the iron-bearing material is sponge iron.

9. The method of claim 7 in which the amount of combined residual oxygen in the sponge iron is between 0.1% and 1.75% in order to produce a vigorous boil in the furnace bath.

10. The method of claim 7 in which the amount of combined residual oxygen in the sponge iron is between 0.7% and 1.2% to produce a vigorous boil in the furnace bath.

11. The method of claim 1 in which the flow of slag over the bath is generated by feeding slag-forming ma- References Cited UNITED STATES PATENTS 3,259,486 7/1966 Kootz 75-52 1 0 FOREIGN PATENTS 289 1878 Great Britain. 685,326 1952 Great Britain. 946,964 1964 Great Britain.

WINSTON A. DOUGLAS, Primary Examiner P. D. ROSENBERG, Assistant Examiner US. Cl. X.R. 10 7530 

